default command introduced.

src/cli.rs

@@ -310,6 +310,7 @@ pub async fn cli() -> Result<(), Box<dyn Error>> {
             per_item_command(Echo),
             whole_stream_command(Config),
             whole_stream_command(Compact),
+            whole_stream_command(Default),
             whole_stream_command(SkipWhile),
             per_item_command(Enter),
             per_item_command(Help),

src/commands.rs

@@ -17,6 +17,7 @@ pub(crate) mod count;
 pub(crate) mod cp;
 pub(crate) mod date;
 pub(crate) mod debug;
+pub(crate) mod default;
 pub(crate) mod echo;
 pub(crate) mod enter;
 pub(crate) mod env;
@@ -105,6 +106,7 @@ pub(crate) use count::Count;
 pub(crate) use cp::Cpy;
 pub(crate) use date::Date;
 pub(crate) use debug::Debug;
+pub(crate) use default::Default;
 pub(crate) use echo::Echo;
 pub(crate) use enter::Enter;
 pub(crate) use env::Env;

src/commands/default.rs

@@ -0,0 +1,72 @@
+use crate::commands::WholeStreamCommand;
+use crate::errors::ShellError;
+use crate::parser::CommandRegistry;
+use crate::prelude::*;
+
+#[derive(Deserialize)]
+struct DefaultArgs {
+    column: Tagged<String>,
+    value: Tagged<Value>,
+}
+
+pub struct Default;
+
+impl WholeStreamCommand for Default {
+    fn name(&self) -> &str {
+        "default"
+    }
+
+    fn signature(&self) -> Signature {
+        Signature::build("default")
+            .required("column name", SyntaxShape::String, "the name of the column")
+            .required(
+                "column value",
+                SyntaxShape::Any,
+                "the value of the column to default",
+            )
+    }
+
+    fn usage(&self) -> &str {
+        "Sets a default row's column if missing."
+    }
+
+    fn run(
+        &self,
+        args: CommandArgs,
+        registry: &CommandRegistry,
+    ) -> Result<OutputStream, ShellError> {
+        args.process(registry, default)?.run()
+    }
+}
+
+fn default(
+    DefaultArgs { column, value }: DefaultArgs,
+    RunnableContext { input, .. }: RunnableContext,
+) -> Result<OutputStream, ShellError> {
+    let stream = input
+        .values
+        .map(move |item| {
+            let mut result = VecDeque::new();
+
+            let should_add = match item {
+                Tagged {
+                    item: Value::Row(ref r),
+                    ..
+                } => r.get_data(&column.item).borrow().is_none(),
+                _ => false,
+            };
+
+            if should_add {
+                match item.insert_data_at_path(&column.item, value.item.clone()) {
+                    Some(new_value) => result.push_back(ReturnSuccess::value(new_value)),
+                    None => {}
+                }
+            } else {
+                result.push_back(ReturnSuccess::value(item));
+            }
+            result
+        })
+        .flatten();
+
+    Ok(stream.to_output_stream())
+}